Alfvén wave dissipation and topological properties
of 3D coronal force-free magnetic fields

Malara, F.; Veltri, P.; Franceschis, M. F. De

doi:10.1051/0004-6361:20066460

Cited by 15 publications

(20 citation statements)

References 38 publications

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“…Since Alfvénic perturbations propagate along magnetic lines, at separatrices the perturbation pattern is strongly stretched. Malara et al (2007) have shown that the rate of small-scale generation is particularly high at such locations. We will show that smallscale structures in the perturbation are located at separatrices.…”

Section: Equilibrium Structurementioning

confidence: 99%

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Alfvén waves in coronal holes: formation of discontinuities in inhomogeneous magnetic fields

Malara

2012

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Context. Solar wind fluctuations are characterized by discontinuities. The nature and properties of these structures have been largely studied in the literature, and different mechanisms have been proposed to explain their formation. Aims. We investigate the evolution of Alfvénic perturbations propagating in the inhomogeneous magnetic field of a coronal open-field region, in order to study both the way that small-scale structures are generated and the possible formation of discontinuities. Methods. We constructed a model for the equilibrium magnetic field in a coronal hole. The model represents a potential field with a complex structure: regions of opposite polarity or of only the dominant polarity are present at low or high altitudes, respectively. The evolution of small-amplitude Alfvén waves in the inhomogeneous structure is studied by employing a WKB approach that describes how the perturbation wavevector and the wave phase vary along magnetic lines. Results. We find that small-scale structures form in the perturbation at relatively low altitudes (∼3 × 10 4 km) above the coronal base. An initially monochromatic perturbation develops a steep power-law spectrum with slope α 2.3, which is strongly anisotropic with a predominance of quasi-perpendicular wavevectors. Small-scale structures are localized around separatrices of the magnetic structures. In many cases they contain quasi-perpendicular rotational discontinuities that can propagate to the upper corona, eventually reaching the solar wind. Conclusions. The considered mechanism could be responsible for forming a fraction of the population of discontinuities detected in the solar wind.

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Section: Equilibrium Structurementioning

confidence: 99%

“…A more complex topology of magnetic field lines corresponds to a more efficient generation of small scales (Malara et al 2007). In particular, Malara et al (2005) consider a complex magnetic configuration that represents a model for the coronal field above a quiet-Sun region.…”

Section: Introductionmentioning

confidence: 99%

Alfvén waves in coronal holes: formation of discontinuities in inhomogeneous magnetic fields

Malara

2012

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“…More recent studies have shown that the creation of a dissipative scale can be made exponentially fast (dk/dt ∝ const.·k), with the associated time scale being proportional to ln S S 1/3 S (Similon & Sudan 1989;Petkaki et al 1998;Malara et al 2007). The latter improved efficiency of the dissipation results by essentially considering a more complex magnetic geometry, with the property of local exponential separation or divergence of neighboring magnetic field lines.…”

Section: Introductionmentioning

confidence: 99%

“…In this case, the diffusive length scale is proportional to S −1/2 . Therefore, fast dissipation is possible in three-dimensional chaotic magnetic fields (Similon & Sudan 1989;Petkaki et al 1998;Malara et al 2000Malara et al , 2007 and in twodimensional regular ones (Smith et al 2007;Malara et al 2003;Ruderman et al 1998), provided they possess a degree of complexity.…”

Section: Introductionmentioning

confidence: 99%

“…Hence, the wave energy spectrum F(k, t) is the sum of the energy of each individual wave packet, with associated wave vector and position, whose time dependence is given by Hamiltonian ray equations. This WKB method has already been employed by several authors (Petkaki et al 1998;Malara et al 2000Malara et al , 2007 to study shear Alfven waves propagation and dissipation in different types of coronal magnetic structures. Our study is a continuation of these works.…”

Section: Introductionmentioning

confidence: 99%

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Mixing of shear Alfven wave packets

Bian¹,

Tsiklauri

2008

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Aims. We consider the propagation of shear Alfven wave packets in inhomogeneous magnetic fields which is at the origin of their distortion regardless of any non-linear coupling. It is shown that this can be regarded as a mixing process and hence, standard phase mixing corresponds to the effect of an Alfvenic shear flow, while enhanced dissipation at a magnetic X-point corresponds to mixing by an Alfvenic strain flow. Methods. The evolution of the wave field is supposed to result from the dynamics of a superposition of wave packets, and a kinetic equation for the wave energy is obtained following this eikonal (WKB) description. Results. Since shear Alfven wave packets experience continuous shearing/straining, while transported by an inhomogeneous Alfvenic flow V A , their mixing process in physical space is also a cascade of wave energy in k-space. The wave energy spectrum resulting from this linear mechanism of energy transfer is determined for the special case of waves propagating along chaotic magnetic field lines, the analog of a chaotic mixing process. The latter follows a k −1 power law in the energy conserving range in k space.

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From Alfvén waves to kinetic Alfvén waves in an inhomogeneous equilibrium structure

et al. 2016

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Kinetic Alfvén waves are believed to primary form fluctuations in a hydromagnetic turbulence at scales of the order of the ion inertial length. We study a model where an initial Alfvén wave propagates inside an equilibrium structure which is inhomogeneous in the direction perpendicular to the equilibrium magnetic field. In a previous paper this situation has been considered in a particular configuration where the initial wave vector is parallel to the magnetic field and the wave is polarized perpendicular to the inhomogeneity direction. Here we consider other configurations, with a different polarization and possible initial oblique propagation. We employ numerical simulations, using both a Hall‐magnetohydrodynamics and a Hybrid Vlasov‐Maxwell model. Results show that in all the considered cases the time evolution leads to the formation of kinetic Alfvén waves within the inhomogeneity regions, which are identified by a comparison with analytical linear theory results. Then, in this context the formation of kinetic Alfvén waves seems to be a general phenomenon which could be also extended to more complex situations, like turbulence. Kinetic simulations show that kinetic Alfvén waves modify the ion distribution function, generating temperature anisotropy of both parallel and perpendicular to the local magnetic field as well as particle beams aligned along the local magnetic field. These results could be relevant both in the solar corona and in large‐scale structures of the solar wind, where Alfvénic fluctuations are present along with large‐scale inhomogeneities.

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Alfvén wave dissipation and topological properties of 3D coronal force-free magnetic fields

Cited by 15 publications

References 38 publications

Alfvén waves in coronal holes: formation of discontinuities in inhomogeneous magnetic fields

Alfvén waves in coronal holes: formation of discontinuities in inhomogeneous magnetic fields

Mixing of shear Alfven wave packets

From Alfvén waves to kinetic Alfvén waves in an inhomogeneous equilibrium structure

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